1,104 research outputs found
A Link Between the Semi-Major Axis of Extrasolar Gas Giant Planets and Stellar Metallicity
The fact that most extrasolar planets found to date are orbiting metal-rich
stars lends credence to the core accretion mechanism of gas giant planet
formation over its competitor, the disc instability mechanism. However, the
core accretion mechanism is not refined to the point of explaining orbital
parameters such as their unexpected semi-major axes and eccentricities. We
propose a model, which correlates the metallicity of the host star with the
original semi-major axis of its most massive planet, prior to migration,
considering that the core accretion scenario governs giant gas planet
formation. The model predicts that the optimum regions for planetary formation
shift inward as stellar metallicity decreases, providing an explanation for the
observed absence of long period planets in metal-poor stars. We compare our
predictions with the available data on extrasolar planets for stars with masses
similar to the mass of the Sun. A fitting procedure produces an estimate of
what we define as the Zero Age Planetary Orbit (ZAPO) curve as a function of
the metallicity of the star. The model also hints that the lack of planets
circling metal-poor stars may be partly caused by an enhanced destruction
probability during the migration process, since the planets lie initially
closer to the central stars.Comment: Nature of the replacement: According to recent simulations, the
temperature profile, T, is more adequately reproduced by beta = 1 rather than
beta = 2. We have introduced a distance scale factor that solves the very
fast drop of T for low metallicity and introduces naturally the inferior
distance limit of our ZAPO. Under this modification all the fitting process
was altere
Spiral structure of the Third Galactic Quadrant and the solution to the Canis Major debate
With the discovery of the Sagittarius dwarf spheroidal (Ibata et al. 1994), a
galaxy caught in the process of merging with the Milky Way, the hunt for other
such accretion events has become a very active field of astrophysical research.
The identification of a stellar ring-like structure in Monoceros, spanning more
than 100 degrees (Newberg et al. 2002), and the detection of an overdensity of
stars in the direction of the constellation of Canis Major (CMa, Martin et al.
2004), apparently associated to the ring, has led to the widespread belief that
a second galaxy being cannibalised by the Milky Way had been found. In this
scenario, the overdensity would be the remaining core of the disrupted galaxy
and the ring would be the tidal debris left behind. However, unlike the
Sagittarius dwarf, which is well below the Galactic plane and whose orbit, and
thus tidal tail, is nearly perpendicular to the plane of the Milky Way, the
putative CMa galaxy and ring are nearly co-planar with the Galactic disk. This
severely complicates the interpretation of observations. In this letter, we
show that our new description of the Milky Way leads to a completely different
picture. We argue that the Norma-Cygnus spiral arm defines a distant stellar
ring crossing Monoceros and the overdensity is simply a projection effect of
looking along the nearby local arm. Our perspective sheds new light on a very
poorly known region, the third Galactic quadrant (3GQ), where CMa is located.Comment: 5 pages, 2 figures. Quality of Fig 1 has been degraded to make it
smaller. Original fig. available on request. accepted for publication in
MNRAS letter
The graphene sheet versus the 2DEG: a relativistic Fano spin-filter via STM and AFM tips
We explore theoretically the density of states (LDOS) probed by an STM tip of
2D systems hosting an adatom and a subsurface impurity,both capacitively
coupled to AFM tips and traversed by antiparallel magnetic fields. Two kinds of
setups are analyzed, a monolayer of graphene and a two-dimensional electron gas
(2DEG). The AFM tips set the impurity levels at the Fermi energy, where two
contrasting behaviors emerge: the Fano factor for the graphene diverges, while
in the 2DEG it approaches zero. As result, the spin-degeneracy of the LDOS is
lifted exclusively in the graphene system, in particular for the asymmetric
regime of Fano interference. The aftermath of this limit is a counterintuitive
phenomenon, which consists of a dominant Fano factor due to the subsurface
impurity even with a stronger STM-adatom coupling. Thus we find a full
polarized conductance, achievable just by displacing vertically the position of
the STM tip. To the best knowledge, our work is the first to propose the Fano
effect as the mechanism to filter spins in graphene. This feature arises from
the massless Dirac electrons within the band structure and allows us to employ
the graphene host as a relativistic Fano spin-filter
Modelo de hargreaves-samani ajustado as condições climáticas do estado do Rio de Janeiro para estimativa da evapotranspiração de referência.
Utilizando-se como padrão para determinação da evapotranspiração de referência (ETo) o modelo de Penman-Monteith parametrizado no boletim Nº 56 de irrigação e drenagem da FAO (PM-FAO56) ajustou-se o modelo de Hargreaves-Samani (HS) para as condições climáticas do Estado do Rio de Janeiro. Nas estimativas da ETo foram utilizadas séries climáticas de 10 estações meteorológicas convencionais pertencentes ao Instituto Nacional de Meteorologia. Os valores dos coeficientes do modelo de HS ajustados para o Estado do Rio de Janeiro não variaram significativamente (cv 3,4%), sendo indicado a utilização de um valor médio (0,0116 ± 0,0004) para todo o Estado. O modelo de Hargreaves-Samani ajustado e o original apresentaram elevada precisão (r2 = 0,94) das estimativas de ETo. Contudo, a acurácia das estimativas de ETo foram superiores com o modelo HS ajustado (d = 0,99 e EPE = 0,25 mm d-1) em relação ao original (d = 0,93 e EPE = 0,65 mm d-1)
Weak insensitivity to initial conditions at the edge of chaos in the logistic map
We extend existing studies of weakly sensitive points within the framework of
Tsallis non-extensive thermodynamics to include weakly insensitive points at
the edge of chaos. Analyzing tangent points of the logistic map we have
verified that the generalized entropy with suitable entropic index q correctly
describes the approach to the attractor.Comment: 6 pages, 3 figure
Compactness in the Thermal Evolution of Twin Stars
In this work, we study for the first time the thermal evolution of twin star
pairs, i.e., stars that present the same mass but different radius and
compactness. We collect available equations of state that give origin to a
second branch of stable compact stars with quarks in their core. For each
equation of state, we investigate the particle composition inside stars and how
differently each twin evolves over time, which depends on the central
density/pressure and consequent crossing of the threshold for the Urca cooling
process. We find that, although the general stellar thermal evolution depends
on mass and particle composition, withing one equation of state, only twin
pairs that differ considerably on compactness can be clearly distinguished by
how they cool down
Assessment of prescribing pattern and cost analysis of topical steroids for skin disorders in dermatological outpatient department of a tertiary care hospital
Background: Topical steroid is most commonly prescribed in non-infective dermatological conditions. Periodical Prescription audit is mandatory for the effective management. Hence the present study is designed to assess the prescribing pattern and cost analysis of topical steroids for various skin disorders in the dermatology OPD of a teaching hospital.Methods: This is a prospective observational study conducted in dermatology OPD of a teaching hospital from Jan-Mar 2019 in patients prescribed with topical steroids in all age groups after obtaining IEC approval. Data was analyzed for prescription pattern and cost analysis using descriptive statistics and expressed in percentage.Results: A total of 90 prescriptions were analyzed among which males were 45% and females were 55%, the common indications were eczema-27.7%, atopic dermatitis-25.5%, psoriasis-16.6%, dermatoses-13.3%, lichen planus-7.77% etc. Topical steroids commonly prescribed were super potent (Clobetasol 34.4% and Halobetasol 22.2%). Cream and ointment formulation were commonly used. Common adverse reactions were skin atrophy, hypopigmentation, acne. In prescribing pattern, specification of strength and quantity were lacking whereas instructions regarding area of application-44.4%, route of administration-83.3%, frequency and duration-91% were noted. In cost analysis, comparison is made between similar potency and clinical outcome. Of which, Clobetasol 0.05% and Betamethasone 0.01% is found to be cost effective compared to Halobetasol 0.05% and Mometasone 0.1% respectively.Conclusions: This study provides a limelight on prescribing pattern of topical steroids and emphasize periodic audit to rationalize the prescription with cost effectiveness
Metallic-insulator phase transitions in the extended Harper model
In this work we investigate the transport properties of non-relativistic
quantum particles on incommensurate multilayered structures with the
thicknesses of the layers following an extended Harper model given by
. For the normal incidence case, which means
an one-dimensional system, we obtained that for a specific range of energy, it
is possible to see a metallic-insulator transition with the exponent . A
metallic phase is supported for . We also obtained that for the specific
value there is an alternation between metallic and insulator phases as
we change the disorder strength . When we integrate out all incidence
angles, which means a two-dimensional system, the metallic-insulator transition
can be seen for much larger range of energy compared to the normal incidence
case
Critical wave-packet dynamics in the power-law bond disordered Anderson Model
We investigate the wave-packet dynamics of the power-law bond disordered
one-dimensional Anderson model with hopping amplitudes decreasing as
. We consider the critical case ().
Using an exact diagonalization scheme on finite chains, we compute the
participation moments of all stationary energy eigenstates as well as the
spreading of an initially localized wave-packet. The eigenstates
multifractality is characterized by the set of fractal dimensions of the
participation moments. The wave-packet shows a diffusive-like spread developing
a power-law tail and achieves a stationary non-uniform profile after reflecting
at the chain boundaries. As a consequence, the time-dependent participation
moments exhibit two distinct scaling regimes. We formulate a finite-size
scaling hypothesis for the participation moments relating their scaling
exponents to the ones governing the return probability and wave-function
power-law decays
On wave interference in planet migration: dead zone torques modified by active zone forcing
We investigate planetary migration in the dead zone of a protoplanetary disk
where there are a set of spiral waves propagating inward due to the turbulence
in the active zone and the Rossby wave instability (RWI), which occurs at the
transition between the dead and active zones. We perform global 3D unstratified
magnetohydrodynamical (MHD) simulations of a gaseous disk with the FARGO3D
code, using weak gradients in the static resistivity profiles that trigger the
formation of a vortex at the outer edge of the dead zone. We find that once the
Rossby vortex develops, spiral waves in the dead zone emerge and interact with
embedded migrating planets by wave interference, which notably changes their
migration. The inward migration becomes faster depending on the mass of the
planet, due mostly to the constructive (destructive) interference between the
outer (inner) spiral arm of the planet and, the destruction of the dynamics of
the horseshoe region by means of the set of background spiral waves propagating
inward. The constructive wave interference produces a more negative Lindblad
differential torque which inevitably leads to an inward migration. Lastly, for
massive planets embedded in the dead zone, we find that the spiral waves can
create an asymmetric wider and depeer gap than in the case of -disks,
and can prevent the formation of vortices at the outer edge of the gap. The
latter could generate a faster or slower migration compared to the standard
type-II migration.Comment: 18 pages, 14 figures, accepted for publication in Ap
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